The read/write heads of a hard disc drive are very sensitive to external shock and vibration. As is well known to those of ordinary skill in the art, shock and vibration dynamics can cause head/disc impact. If the impact occurs in the area of the disc where data is stored, data loss or permanent damage can occur. Typically, there is a maximum shock and vibration specification limit that corresponds to a given head design. Shock specifications are typically given for both operational and non-operational states. Disc drives will typically see higher shock levels during the non-operational state primarily resulting from shipping and handling.
Modern designs incorporate latching or locking mechanisms to hold the actuator arm at a specific position when the drive is not in operation. This allows higher non-operational dynamics to be tolerated and prevents a fatal event, caused by a drop or careless handling that would physically damage the drive. Most current latch designs are passive and require external sources of mechanical energy. For example, certain latch designs take advantage of the high velocity airflow generated by the spinning discs for actuation. Other designs rely on the inertia of a separate member that moves when a shock is imparted to the drive.
Air vane and inertial style latches are likely the most commonly used latches in industry today. Both styles are preferred for their high shock resistance capabilities. Inertial latches are common in drives sized for notebook computers and consumer electronics. The latch consists of a separate inertial member rotating about a designated pivot point. Both the pivot location and inertia of the member are designed so that the member engages the actuator arm in a finite time under the influence of a specified shock. The engagement contact effectively blocks actuator motion and prevents the read/write heads from moving into the data zone. The timing of this engagement can become problematic, inconsistent, or even impossible if wide ranges of shock resistance are required. For example, the latch design may meet a specified upper amplitude limit at the expense of lower amplitudes. Those of ordinary skill in the art will recognize that it is possible for inertial latches to engage the actuator at higher shock amplitudes, but “miss” at lower amplitudes.
Air latches are typically found in desktop and server drives where larger disc sizes and higher spindle speeds provide stronger airflow currents. In a typical configuration, opposite the air vane will be an engagement feature that keeps the actuator arm locked in shipping position when the drive is off. When power is applied to the drive, force is applied to the latch vane as a result of the airflow from spinning discs. The latch overcomes the return bias and stays open as long as power is applied and the discs are spinning.
However, the use of air latches is not without problems. For example, disc drive manufacturers frequently remove discs or “depopulate” an existing drive design to create different capacity points for one set of drive mechanics. This allows the manufacturer to design a single set of mechanics and provide customers with a range of different capacities. However, reducing the disc count will obviously change the airflow characteristics and dynamics of the system and almost invariably results in the need for multiple latch designs. Thus, difficulties can arise in use of a common air latch with a product scheduled for depopulation: multiple latch designs may be required which is, of course, economically undesirable.
Although not as common, some latches are designed to take advantage of magnetic forces that are inherent in the magnetic circuit or supplied by a separate magnet. For example, magnetic bi-stable latches are typically found in high-end server drives where shock requirements are not so stringent. The bi-stable latch is so-called because it has two stable equilibrium points. In a typical bi-stable latch, a plastic member rotates about a designated pivot pin. A magnet is typically molded into the plastic member and is attracted to two separate steel pins, with the size and positioning of the magnet being largely determinative of the amount of force produced thereby. The proximity of the magnet determines to which pin the magnet is attracted. A properly designed bi-stable latch is not allowed to rest in between the two pins. The engagement contact dynamics between the latch and actuator arm are analogous to that of two spur gears.
Magnetic latches are not without their problems. First, magnetic, bi-stable latches are not as common as some other types of latches because of an inherent low shock resistance capability. Additionally, a magnetic latch is actuated by the disc drive actuator itself so that the latch holding torque requirement is at odds with the arm opening torque requirement, e.g., the disc drive actuator must overcome the latch holding force when the drive is powered on. Further, most bi-stable latches are positioned behind the actuator, which tends to limit the coil length and, of course, a longer coil generally results in a higher torque capability of the actuator. Also, because since the latching process inevitably involves some impact, the potential exists for particulate generation which can be fatal to a disc drive. Finally, in order to resist most shocks, the bias is required to be active for most (or all) of the actuator stroke angle. Typically, the bias torque is nonlinear and significant compared to the available actuator torque, which can have detrimental effects on seek performance and track follow power. The presence of a non-linear magnetic bias while the drive is seeking greatly complicates the theory and practice of moving the disc drive arm. Heretofore, the presence of such bias has resulted in a drive that is much slower to reach a designated track and begin reading than would otherwise be preferred. Prior art approaches to solving this problem have attempted to minimize or eliminate the magnetic force during the time that a seek is underway, but that approach is not without its own problems. Further, the most straightforward approach to improving move-time (i.e., decreasing it) in the presence of a magnetic bias would be to increase the available power. However, customer price and other constraints, and industry standards tend to limit the amount of overall power a given disc drive can consume and the move power factors into that limitation.
The economic advantages of using a pure magnetic bias for shock resistance argue that, in spite of the above-identified and other problems, the use of such latches use should be investigated. Although any number of prior art references have considered this approach, there remains no satisfactory solution to the problems associated with the use of magnetic latches and, in more particular, to the problems associated with moving a disc drive arm in the presence of nonlinear magnetic bias. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.